Mon, Aug 30, 1999
------------------------

- Review of E/R Modeling

- Multiway Relationships
	- why do we need them?
	- notation

- Roles in Relationships
	- when the same entity set appears more than once
          in the same relationship
	- Person married to Person (roles: husband, wife)
	- Person married to Person with Person (child role)

- Symmetric Relationships
	- when for every (a,b), there is also a (b,a)

- Example of Symmetric Relationships
	- Many-Many: Friends (assuming standard laws of the world)
	- One-One: Married (when the edges are labeled spouse,
	instead of husband/wife)

- Question from Akash: Can a many-one relationship be symmetric?

- Answer: No (I incorrectly said "possibly" earlier)
	- Reasoning: Assume a relationship is many-one from A to B.
	Example data: (a1,b1), (a2,b1), (a3,b1). 
	If we assume it is symmetric, we should also have the tuples
	(b1,a1), (b1,a2), (b1,a3)
	but then, it is many-one in the reverse direction.

	- Conclusion: Only many-many or one-one can be symmetric

- Attributes on Relationships
	- when something belongs to the relationship rather than
	either of the entity sets participating in it
	- Motivation: divorce proceedings etc. :-)

- Removing multi-way relationships, roles, and attributes from relationships
	- use a connecting entity set and introduce new many-one
	relationships

- Sometimes, even the relationship might have "identity" in the real world:
	- Example: Movies, Actors and Contracts.
	- Contracts, though a relationship, has meaning as a "thing"
	in life

 

Wed, Sep 1, 1999
------------------------

- Review of converting multiway to two-way relationships

- Good Design Principles
	- Be faithful
	- Avoid redundancy
	- KISS
	- Pick the right kind of element
	  (Contracts example)

- Inheritance in E/R Modeling
	- Use triangles to denote ISA links
	- Entity can belong to more than one entity set 
          simultaneously

- Constraints:
	- Key constraints 
		- use underlines
	- Referential Integrity (stricter than a -> one constraint)
		- use a rounded arrow entering the "important" set
		- works only on two-sets at a time
		- for more complicated ref. integrity constraints,
	          need to encode it in application
	- Referential Integrity = -> + "has to exist"
	- If the "important object" is deleted
		- delete the "other object" too!
	- If a new "other object" is introduced
		- connect it to an existing "important object"

- For things un-modellable by E/R
	- Write "Note: ... " on the margin of the page! :-)

Fri, Sep 3, 1999
------------------------

- Weak Entity Sets

- Causes of Weak Sets:
	- Elimination of Multiway relationships
	- Hierarchy of Entity Sets

- Solution of an University Database Problem (handout provided in class)